Digital sonar technology and submersed aquatic vegetation (SAV) analysis software were demonstrated in Roche Harbor, San Juan Island to determine their effectiveness for characterizing eelgrass (Zostera marina) in San Juan County. A BioSonics 420 kHz echosounder system was tested in a variety of habitats and obtained comparable eelgrass distributions to those acquired from diver surveys. The two dominant submersed aquatic vegetation (SAV) species present, eelgrass and sea lettuce (Ulva lactica), were generally easy to distinguish from one another by their acoustic signal. A map of a surveyed area off Henry Island was produced using BioSonics’ EcoSAV software and a commercial mapping package. This remote sensing system appears to offer a high speed, high resolution means for surveying eelgrass throughout San Juan County.

A recently developed remote sensing technology, which utilizes a digital echosounder system to characterize and map aquatic vegetation, was demonstrated in Roche Harbor on San Juan Island on May 5, 2001 to determine whether or not this technology can effectively map eelgrass communities in San Juan County. The remote sensing system includes an ultrasonic frequency BioSonics echosounder system, a Differential Global Positioning System (DGPS) and PC laptop for data acquisition and analysis.

The BioSonics digital echosounder system is made up of two components: the transducer, which is placed in the water, and the deck unit, which houses the electronic components. The laptop is used to control the echosounder and acquire data. While collecting data, a real time echogram and positional data are available. The echogram can be used for information such as plant presence/absence, plant species and bottom depth. Data analysis is performed with BioSonics’ EcoSAV software. The software produces an ASCII output with ping number, DGPS position (including time and date), bottom depth, aquatic plant coverage and average aquatic plant height.

Three primary areas near Roche Harbor, San Juan Island were surveyed: Mosquito Pass, Open Bay and Henry Island. Divers from Puget Sound Bio Survey had previously surveyed these three areas and eelgrass maps were available. The digital sonar technique was tested by comparing locations where eelgrass was detected to the same locations on these maps.

Distinct differences between the acoustic signal from eelgrass and sea lettuce were observed at most locations. Few difficulties were encountered in the field in detecting eelgrass boundaries in Mosquito Pass. Where thick sea lettuce and eelgrass communities were located next to one another, acoustic differences were observed. In Open Bay, where low-density eelgrass communities were observed to be integrated with sea lettuce, eelgrass detection was reliable and verified with the underwater video system. Near Henry Island, very tall, dense stands of eelgrass were observed and easily distinguished from the little amount of sea lettuce that was observed. A large stand of eelgrass was detected that extended out to the main channel until the point where the sea bottom became too deep to colonize. In the dense beds, most eelgrass was approximately 1m tall, though some grew as tall as 1.6m on the west side of the survey area. Small patches of less than 3-5m diameter were detected, as well as a distinct absence of eelgrass from the seabed adjacent to the pier.

The digital sonar successfully characterized eelgrass in the areas surveyed. Eelgrass boundaries detected by the system were similar to those detected by divers. The quality of data resolution was also comparable to diver surveys; eelgrass patches of 3-5m diameter were detected. At lower boat speeds, the system can detect patches less than a meter in diameter. Generally, strong differences between the acoustic signal from eelgrass and sea lettuce were observed. The ability to perform work in both intertidal and subtidal regions at high resolution is a strong advantage to this system. Further advantages include the ease of portability, user-friendly software, relatively fast data analysis, relatively low-cost operational expenses and ease of integration with GIS applications. Overall, it appears that this technology would be a valuable tool for comprehensive mapping of eelgrass throughout San Juan County.

The basis for acoustical bathymetric surveys is detecting and timing the echo from a short, vertically oriented pulse. The exact detection process may vary from system to system but is usually based on exceedence of some minimum threshold intensity and peak width. For bathymetric surveys of navigation channels, this approach usually works well. A typical navigation channel consists of open water above a distinct sediment interface, leading to no ambiguity in relating the time of the echoed pulse to the exact depth of the sediment interface. A decided exception to this occurs when the bottom is colonized with submersed aquatic vegetation. Under these conditions, the acoustical reflectivity of the gas-filled plant stems or blades generates an echo that arrives at the receiver before the true bottom echo. Depending on plant type, height, and density, these plant-generated returns may pass the test for the detected bottom and be declared as the bottom, underestimating the true depth. If undetected, this condition can lead to erroneous surveys of channel depth and overestimates of dredging quantities required to keep the channel at its authorized depth.

While this occurs in only a small percentage of the channels maintained by the U.S. Army Corps of Engineers, it is sufficiently common in certain regions to represent a major operational problem. A common “offending” plant species is Zostera marina (eelgrass), which occurs in cool, clear, shallow saltwater locations along much of the northeastern and Pacific coastline of the United States. Approximately 60 small boat harbors within the Corps’ New England District have eelgrass established within the project bounds. Hydrographic surveying within these areas requires extra field work to properly identify the true bottom. Additional data processing and field checking are necessary to verify the existence of the eelgrass and to ascertain that the bottom has been successfully tracked. This simply causes extra work at locations which have a known history of eelgrass. The major concern occurs at locations where eelgrass presence is not suspected. Here, eelgrass presence may go undetected and can cause both an environmental problem and errors in estimated dredging quantities.

During the summer of 1998, a bathymetric condition survey in an eelgrass-infested channel (Wood Island Harbor) was conducted simultaneously using two very different hydroacoustic depth measurement systems. The first was an Odom EchoTrac 3200 MKII (Odom Hydrographic, Baton Rouge, LA) with a 200-kHz, 8-deg transducer, a widely used hydrographic system. The second system was the Submersed Aquatic Vegetation Early Warning System (SAVEWS), which uses the Biosonics DT Series single beam digital echosounder (Biosonics Inc., Seattle, WA) with a 420-kHz, 6-deg transducer. SAVEWS (Sabol and Burczinski 1998) is specifically designed to detect submersed vegetation and measure canopy density and height. Analyses of the resulting data showed good agreement between depth estimates from the two systems in unvegetated areas but increasing disagreement as eelgrass density increased. This disagreement was thought to be the result of primarily the differing signal processing approaches used. A short exploratory study was conducted of alternative processing approaches using a sampling of the digital DT Series Echosounder data. Each of these aspects is discussed in the full report (see document link below) and evidence is presented that improved bottom tracking within vegetated areas can be achieved using existing sensor hardware with a modified signal processing approach.

A surface of roughly 4sqkm has been covered with transects spaced 20 m on average, resulting in a total track length of about 250 km, taking some 800,000 samples (pings).

A pre-release version of a specific software, based on SAVEWS (Submersed Aquatic Vegetation Early Warning System), developed by Bruce Sabol, USACE Waterways Experiment Station, Vicksburg, and currently under further development through BioSonics, Inc., Seattle, was deployed in order to process the split beam raw data (only single beam data is read by the program).

Previous to the survey, a number of fixed position observations over a frame (50x50cm) were done in order to dispose of data for calibration purposes. Afterwards the frame area has been fully sampled physically.

Data on existence of plants as detected by the program was verified comparing the findings with the echograms. The percentage of plant detection over a cycle of 8 pings as a measure for the density or coverage of detected plants and plant mean height are also available from the program output. These parameters have been interpolated and subsequently presented on maps. Additionally, ground truth data from 100 physical field samples is available to verify findings.

The desire to distinguish the two species present in the area, Zostera marina and Zostera noltii, was not yet achieved based on the obtained data. Due to the specific configuration of the equipment during data acquisition, TS analysis with common tools are not easily done. An empirical approach to separate species based on a combination of height and depth has not yet concluded.

Finally, a series of problems, as for example the inclination of plants due to currents, are discussed.

Offshore Scientific Services completed a study, using a BioSonics echosounder, which evaluated an acoustic method for determining the density and location of seagrass beds in a given area. These tests were performed in Narrabeen Lagoon, NSW Australia in April 1994. The species analyzed for this study was Zostera capricorni. A technical evaluation of this method is important for determining the repeatability, resolution and accuracy of calibration of this technique.

The benefits of using a successful acoustic method to map seagrass density and location include the following: 1.)Technique is insensitive to turbidity, depth and surface roughness 2.)Direct measure of density obtained in the field 3.)Method yields data that is high in resolution and repeatable in nature 4.)Large areas can be surveyed quickly and results output in a short period of time
5.)Reduced reliance on manpower-intensive ground truthing and manual plant-counting exercises.

This project was divided into three phases, as follows: Phase I-Ground truth area of seagrass to compare with acoustic results. Phase II-Calibrate acoustic system in order to convert acoustic signals into seagrass density estimates. Phase III-Use BioSonics acoustic system to map area covered by ground truthing and compare results.

In summary, this study shows the success of this method for obtaining seagrass bed location and density. This application has been shown to provide high quality, quantitative measures of the characteristics of seagrass beds on a variety of scales. The method was also shown to be repeatable and to give reliable estimates of species density.

(Full report is no longer available. For more information, please email sales@biosonicsinc.com)